A magnetic random access memory (MRAM) is a non-volatile memory which basically includes a giant magnetoresistive (GMR) material or magnetic tunneling junction (MTJ) structure, a sense line, and a word line. The MRAM employs the magnetic vectors to store memory states. Magnetic vectors in one or all of the layers of GMR material or MTJ are switched very quickly from one direction to an opposite direction when a magnetic field is applied to the GMR material or MTJ over a certain threshold. According to the direction of the magnetic vectors in the GMR material or MTJ, states are stored, for example, one direction can be defined as a logic "0", and another direction can be defined as a logic "1". The GMR material or MTJ maintains these states even without a magnetic field being applied. The states stored in the GMR material or MTJ can be read by passing a sense current through the cell in a sense line because of the difference between the resistances of the two states.
Magnetic tunneling junction (MTJ) structure or cells include at least a pair of magnetic layers with a non-magnetic, non-conducting tunnel layer sandwiched therebetween. When a sense voltage is applied between the pair of magnetic layers, electrical carriers travel between the pair of magnetic layers by tunneling through the non-magnetic, non-conducting tunnel layer sandwiched between the magnetic layers. The resistance to the sense current is a maximum when the magnetic vectors of the pair of magnetic layers are anti-parallel and minimum when the magnetic vectors of the pair of magnetic layers are parallel. The difference between the maximum and minimum resistance is commonly referred to as the magnetoresistance (MR) ratio.
Further, the minimum resistance of the MTJ cell (commonly referred to as the areal resistance) is determined by the construction and materials of the MTJ cell. Clearly, in an ideal MTJ cell the areal resistance would be very low or zero and the maximum resistance would be very high or infinite, similar to an ideal switch. Prior art attempts to reduce the areal resistance include depositing a layer of pure aluminum on the lower magnetic layer and then oxidizing the aluminum layer in oxygen plasma. A problem with this procedure is that as the aluminum layer is deposited, pinholes tend to form, especially if the layer is thin. As the aluminum is oxidized, some of the pinholes tend to remain and produce shorts in the MTJ cell when the second magnetic layer is deposited on the aluminum oxide layer. To overcome the pinhole problem, one possible solution is to deposit the aluminum layer at low temperatures (e.g. the temperature of liquid nitrogen) for reducing the size of the grains. Some of the problems with this method are that it involves extensive heating and cooling cycles, takes a long time, costs more and hence is not a method which can be used in manufacturing.
Accordingly, it would be highly advantageous if MTJ cells could be fabricated at room temperature without the problem of pinholes and the like.
It is a purpose of the present invention to provide a new and improved method of fabricating MTJ cells with reduced areal resistance.
It is another purpose of the present invention to provide a new and improved method of fabricating MTJ cells with high quality barriers or tunnel layers.
It is a still another purpose of the present invention to provide a new and improved method of fabricating MTJ cells with high magnetoresistance ratios.
It is a further purpose of the present invention to provide a new and improved method of fabricating MTJ cells with thinner and continuous metal layers for the formation of the barrier or tunnel layer.
It is still a further purpose of the present invention to provide a new and improved method of fabricating MTJ cells which does not require extensive cooling and heating cycles and which is easily adaptable to manufacturing.